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<meta name="description" content="根据网上的说法，jdk并发包中的 Condition await 与 signal 实现了 Object.wait notify 语义。以下总结，是基于Condition await,singal方法的实现原理总结出来的：  monitorObject.wait，该方法调用必须在临界区中（锁保护的代码段）被调用，线程如果在临界区中调用监视器的wait方法，然后线程会释放占有监视器monitorOb">
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<meta property="og:description" content="根据网上的说法，jdk并发包中的 Condition await 与 signal 实现了 Object.wait notify 语义。以下总结，是基于Condition await,singal方法的实现原理总结出来的：  monitorObject.wait，该方法调用必须在临界区中（锁保护的代码段）被调用，线程如果在临界区中调用监视器的wait方法，然后线程会释放占有监视器monitorOb">
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          <h1 class="post-title" itemprop="name headline">java并发锁机制-ReentrantLock Condtion准备篇</h1>
        

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        <div id="vip-container"><p>根据网上的说法，jdk并发包中的 Condition await 与 signal 实现了 Object.wait notify 语义。以下总结，是基于Condition await,singal方法的实现原理总结出来的：</p>
<ul>
<li>monitorObject.wait，该方法调用必须在临界区中（锁保护的代码段）被调用，线程如果在临界区中调用监视器的wait方法，然后线程会释放占有监视器monitorObject的锁，然后阻塞（等待条件的发生，该线程会保存在monitorObject的条件等待队列，当该线程收到信号或中断被唤醒后，首先需要尝试获取监视器的锁，然后继续执行操作，如果是被中断，需要在获取锁后，才会被中断。）</li>
<li>monitorObject.notify,该方法调用同样只能在临界区中被调用，锁的释放，在执行完临界区后，才会释放。根据Condition.singal实现机制，首先唤醒，是先将线程从条件等待队列放入到同步阻塞队列，然后执行完临界区代码后，释放锁，其他线程竞争锁。</li>
</ul>
<p>为了对Condition await 与 signal 方法有一个直接的了解，现给出一个简单的生产者、消费者测试示例：</p>
<span id="more"></span>

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class="keyword">import</span> java.util.concurrent.locks.Lock;</span><br><span class="line"><span class="keyword">import</span> java.util.concurrent.locks.ReentrantLock;</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">TestMain</span> </span>&#123;</span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</span><br><span class="line">		BreadContainerByObject container = <span class="keyword">new</span> BreadContainerByObject();</span><br><span class="line">		<span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i &lt; <span class="number">5</span>; i++) &#123;</span><br><span class="line">			<span class="keyword">new</span> Thread(<span class="keyword">new</span> Producers(container)).start();</span><br><span class="line">		&#125;</span><br><span class="line">		<span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i &lt; <span class="number">3</span>; i++) &#123;</span><br><span class="line">			<span class="keyword">new</span> Thread(<span class="keyword">new</span> Customer(container)).start();</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="class"><span class="keyword">interface</span> <span class="title">BreadContainer</span> <span class="keyword">extends</span> <span class="title">Serializable</span> </span>&#123;</span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">put</span><span class="params">(Bread b)</span> <span class="keyword">throws</span> InterruptedException</span>;</span><br><span class="line">	<span class="function"><span class="keyword">public</span> Bread <span class="title">poll</span><span class="params">()</span> <span class="keyword">throws</span> InterruptedException</span>;</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * 基于 Reentrant Condition实现</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@author</span> dingwei2</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="meta">@SuppressWarnings(&quot;serial&quot;)</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">BreadContainerByCondition</span> <span class="keyword">implements</span> <span class="title">BreadContainer</span> </span>&#123;</span><br><span class="line">	<span class="keyword">private</span> Lock lock = <span class="keyword">new</span> ReentrantLock();</span><br><span class="line">	<span class="keyword">private</span> Condition NotFull = lock.newCondition();</span><br><span class="line">	<span class="keyword">private</span> Condition NotEmpty = lock.newCondition();</span><br><span class="line">	<span class="comment">// 面包容器</span></span><br><span class="line">	<span class="keyword">private</span> List&lt;Bread&gt; breads = <span class="keyword">new</span> ArrayList&lt;Bread&gt;();</span><br><span class="line">	<span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> <span class="keyword">int</span> MAX = <span class="number">20</span>;</span><br><span class="line">	<span class="keyword">private</span> <span class="keyword">volatile</span> <span class="keyword">int</span> num = <span class="number">0</span>;</span><br><span class="line">	<span class="meta">@Override</span></span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">put</span><span class="params">(Bread b)</span> <span class="keyword">throws</span> InterruptedException </span>&#123;</span><br><span class="line">		<span class="comment">// TODO Auto-generated method stub</span></span><br><span class="line">		<span class="keyword">try</span> &#123;</span><br><span class="line">			lock.lock();</span><br><span class="line">			<span class="keyword">while</span>(breads.size() &gt;= MAX ) &#123; <span class="comment">//已经满了</span></span><br><span class="line">				NotFull.await();</span><br><span class="line">			&#125;</span><br><span class="line">			b.setId(num ++);</span><br><span class="line">			breads.add(b);</span><br><span class="line">			<span class="comment">//放入一个元素后，NotEmpty</span></span><br><span class="line">			NotEmpty.signalAll();</span><br><span class="line">		&#125; <span class="keyword">finally</span> &#123;</span><br><span class="line">			lock.unlock();</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="meta">@Override</span></span><br><span class="line">	<span class="function"><span class="keyword">public</span> Bread <span class="title">poll</span><span class="params">()</span> <span class="keyword">throws</span> InterruptedException</span>&#123;</span><br><span class="line">		<span class="keyword">try</span> &#123;</span><br><span class="line">			lock.lock();</span><br><span class="line">			<span class="keyword">while</span>(breads.isEmpty()) &#123;<span class="comment">//如果为空</span></span><br><span class="line">				NotEmpty.await();</span><br><span class="line">			&#125;</span><br><span class="line">			Bread b = breads.remove(breads.size() -<span class="number">1</span>);</span><br><span class="line">			NotFull.signalAll();</span><br><span class="line">			<span class="keyword">return</span> b;	</span><br><span class="line">		&#125; <span class="keyword">finally</span> &#123;</span><br><span class="line">			lock.unlock();</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * 基于 Object.notify Object.wait</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@author</span> dingwei2</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="meta">@SuppressWarnings(&quot;serial&quot;)</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">BreadContainerByObject</span> <span class="keyword">implements</span> <span class="title">BreadContainer</span></span>&#123;</span><br><span class="line">	<span class="comment">// 面包容器</span></span><br><span class="line">	<span class="keyword">private</span> List&lt;Bread&gt; breads = <span class="keyword">new</span> ArrayList&lt;Bread&gt;();</span><br><span class="line">	<span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> <span class="keyword">int</span> MAX = <span class="number">20</span>;</span><br><span class="line">	<span class="keyword">private</span> <span class="keyword">volatile</span> <span class="keyword">int</span> num = <span class="number">0</span>;</span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">put</span><span class="params">(Bread b)</span> </span>&#123;</span><br><span class="line">		<span class="keyword">synchronized</span> (breads) &#123;</span><br><span class="line">			<span class="keyword">while</span>(breads.size() &gt;= MAX) &#123;</span><br><span class="line">				<span class="keyword">try</span> &#123;</span><br><span class="line">					breads.wait();</span><br><span class="line">				&#125; <span class="keyword">catch</span> (InterruptedException e) &#123;</span><br><span class="line">					<span class="comment">// TODO Auto-generated catch block</span></span><br><span class="line">					e.printStackTrace();<span class="comment">//这里不应该 将 InterruptedExcepiton 吞掉</span></span><br><span class="line">				&#125;</span><br><span class="line">			&#125;</span><br><span class="line">			b.setId(num ++);</span><br><span class="line">			breads.add(b);</span><br><span class="line">			breads.notifyAll();</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">	<span class="function"><span class="keyword">public</span> Bread <span class="title">poll</span><span class="params">()</span> </span>&#123;</span><br><span class="line">		Bread b = <span class="keyword">null</span>;</span><br><span class="line">		<span class="keyword">synchronized</span> (breads) &#123;</span><br><span class="line">			<span class="keyword">while</span>(breads.size() &lt; <span class="number">1</span>) &#123;</span><br><span class="line">				<span class="keyword">try</span> &#123;</span><br><span class="line">					breads.wait();</span><br><span class="line">				&#125; <span class="keyword">catch</span> (InterruptedException e) &#123;</span><br><span class="line">					<span class="comment">// TODO Auto-generated catch block</span></span><br><span class="line">					e.printStackTrace();<span class="comment">//这里不应该 将 InterruptedExcepiton 吞掉</span></span><br><span class="line">				&#125;</span><br><span class="line">			&#125;</span><br><span class="line">			b = breads.remove(breads.size() -<span class="number">1</span>);</span><br><span class="line">			breads.notifyAll();</span><br><span class="line">		&#125;</span><br><span class="line">		<span class="keyword">return</span> b;</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * 生产者</span></span><br><span class="line"><span class="comment"> * </span></span><br><span class="line"><span class="comment"> * <span class="doctag">@author</span> dingwei2</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Producers</span> <span class="keyword">implements</span> <span class="title">Runnable</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">	<span class="keyword">private</span> BreadContainerByObject container;</span><br><span class="line"></span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="title">Producers</span><span class="params">(BreadContainerByObject container)</span> </span>&#123;</span><br><span class="line">		<span class="keyword">this</span>.container = container;</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="meta">@Override</span></span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">run</span><span class="params">()</span> </span>&#123;</span><br><span class="line">		<span class="comment">// TODO Auto-generated method stub</span></span><br><span class="line">		<span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i &lt; <span class="number">5</span>; i++) &#123;</span><br><span class="line">			Bread b = <span class="keyword">new</span> Bread();</span><br><span class="line">			b.setFactoryName(Thread.currentThread().getName());</span><br><span class="line">			container.put(b);</span><br><span class="line">		&#125;</span><br><span class="line"></span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * 消费者</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@author</span> dingwei2</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Customer</span> <span class="keyword">implements</span> <span class="title">Runnable</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">	<span class="keyword">public</span> BreadContainerByObject container;</span><br><span class="line"></span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="title">Customer</span><span class="params">(BreadContainerByObject container)</span> </span>&#123;</span><br><span class="line">		<span class="keyword">this</span>.container = container;</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="meta">@Override</span></span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">run</span><span class="params">()</span> </span>&#123;</span><br><span class="line">		<span class="comment">// TODO Auto-generated method stub</span></span><br><span class="line">		<span class="keyword">for</span>(<span class="keyword">int</span> i = <span class="number">0</span>; i &lt; <span class="number">5</span>; i ++ ) &#123;</span><br><span class="line">			Bread b = container.poll();</span><br><span class="line">			System.out.println(Thread.currentThread().getName() + <span class="string">&quot;消费了&quot;</span> + b.toString());</span><br><span class="line">		&#125;</span><br><span class="line">		</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="meta">@SuppressWarnings(&quot;serial&quot;)</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Bread</span> <span class="keyword">implements</span> <span class="title">Serializable</span> </span>&#123;</span><br><span class="line">	</span><br><span class="line">	<span class="keyword">private</span> Integer id;</span><br><span class="line">	</span><br><span class="line">	<span class="keyword">private</span> String factoryName;</span><br><span class="line"></span><br><span class="line">	<span class="function"><span class="keyword">public</span> Integer <span class="title">getId</span><span class="params">()</span> </span>&#123;</span><br><span class="line">		<span class="keyword">return</span> id;</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setId</span><span class="params">(Integer id)</span> </span>&#123;</span><br><span class="line">		<span class="keyword">this</span>.id = id;</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="meta">@Override</span></span><br><span class="line">	<span class="function"><span class="keyword">public</span> String <span class="title">toString</span><span class="params">()</span> </span>&#123;</span><br><span class="line">		<span class="keyword">return</span> <span class="string">&quot;面包:&quot;</span> + (id == <span class="keyword">null</span> ? <span class="number">0</span> : id.intValue()) + <span class="string">&quot;;生产工厂：&quot;</span> + getFactoryName();</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="function"><span class="keyword">public</span> String <span class="title">getFactoryName</span><span class="params">()</span> </span>&#123;</span><br><span class="line">		<span class="keyword">return</span> factoryName;</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setFactoryName</span><span class="params">(String factoryName)</span> </span>&#123;</span><br><span class="line">		<span class="keyword">this</span>.factoryName = factoryName;</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>使用Reentrant Condition ,细化了消息通知的粒度，比如使用了当队列中产品时，通过 NotEmpty 条件来唤醒消费者，当队列还有可用的空间存放产品时，使用 NotFull 条件来唤醒生产者，使用两个条件队列，确保被唤醒的线程的准确性，加入到同步队列的节点，在该节点获取到锁后，确实是满足条件的（特别在临界情况的时候)。而Object.wait, Object.notify ,  生产者，消费者在同一个条件队列中排队。</p>
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